This application is related to and claims priority under 35 U.S.C. xc2xa7 119 to Japanese Patent Application No. HEI 11-332045, filed on Nov. 22, 1999, entitled xe2x80x9cCAN MANUFACTURING APPARATUS, CAN MANUFACTURING METHOD, AND CAN,xe2x80x9d and incorporated by reference herein.
1. Field of the Invention
The present invention relates to a can manufacturing apparatus, a can manufacturing method, and a can, wherein the apparatus for manufacturing the can has a dome section and an annular projection, the dome section being formed on a bottom of the can and is recessed inwardly of a body of the can, and the annular projection being formed around the peripheral edge of the dome section so as to project outwardly in a can axis direction.
2. Discussion of Background
Two-piece aluminum alloy cans composed of a cylindrical bottomed can body is well known. A can end is fixed to an opening in the can body. The can body of the two-piece can is made by, first, stamping and drawing an aluminum alloy blank sheet to form a cup member using a drawing apparatus. Then, the cup member is redrawn and ironed with a punch sleeve inserted therein, while the cup member is held. In this way, the peripheral edge of the bottom portion of the cup member is drawn, while it is clamped between the punch sleeve and a dome molding unit. The dome molding unit has a semi-spherical extreme end and is disposed in opposition to the punch sleeve so as to be coaxially therewith. Thus, a can body 101, having the bottom shape shown in FIG. 13, is obtained.
The can bottom 103 of the can body 101 has a dome section 105 and an annular projection (rim) 107 formed thereon. The dome section 105 is spherically recessed inwardly of the can body 101, and the annular projection (rim) 107 joins the peripheral edge of the dome section 105 and projects outwardly of the can body 101 in a direction of the can axis. The annular projection 107 acts as a leg, which is in contact with the ground, when the can body 101 stands upright. Thus, the standing stability and supporting strength of the can body 101 can be improved.
Today""s trend toward reduced wall thickness of cans to save resources and costs has lead to various disadvantages due to the reduction of the strength of the cans. One disadvantages results from a phenomenon called bottom growth, wherein an annular projection 107 of a can bottom 103 is deformed outwardly in a radial direction, while projecting downwardly due to the action of internal pressure, after contents are packed into the can (see the double dot and dashed line of FIG. 14). One of factors which causes bottom growth is the insufficient rigidity of an inner wall 107a, acting as the inner peripheral wall of the annular projection 107. A first peripheral edge of the inner wall 107a is joined to the dome section 105 through a counter sink R-section 109 to form a concave surface. A second peripheral edge of the inner wall 107a is joined to a nose section 107b to form an extreme end of the annular projection 107. When an internal pressure acts on the can bottom 103, the thin inner wall 107a is stressed in the circumferential direction and in the direction of the can axis. In particular, when the stress (elongation) in the direction of the can axis is increased, the annular projection 107 is deformed downwardly and radially outwardly.
When bottom growth occurs, the total height of a can is increased. Thus, problems may arise due to the can""s increased height, such as the cans may become caught on a conveyor as the can is transported to be packaged or the cans may be difficult to package.
Another problem is that as the thickness of the can is reduced, the falling strength of a can bottom becomes insufficient. In other words, when a can falls, the peripheral portion of the dome section 55 of the can bottom 53, which is a particularly fragile portion when subjected to shock caused by a can""s fall, swells and may be broken in the worst-case scenario.
In view of the above-described disadvantages, an object of the present invention is to provide a can, a can manufacturing apparatus and a can manufacturing method, wherein the apparatus and method are for manufacturing a can having a reduced wall thickness, particularly in the can bottom, yet retaining sufficient can strength so as to prevent deformation of the can bottom and suppress bottom growth.
According to a first aspect of the present invention, the can manufacturing apparatus is for forming a can body having a dome section and an annular projection, wherein the dome section is formed on the can bottom and is recessed inwardly, and wherein the annular projection is formed around the peripheral edge of the dome section and projects outwardly in the direction of the can axis. The annular projection has an inner peripheral wall with a plurality of inner recesses in a circumferential direction. The plurality of recesses are recessed inwardly. The can manufacturing apparatus includes: an apparatus main body; a can body support means, disposed on the apparatus main body for supporting the can body; and a can bottom processing means, disposed on the apparatus main body. The can bottom processing means moves relatively in the direction of the can axis with respect to the can bottom. The can processing means includes: a plurality of punch pawls disposed in the circumferential direction, wherein each punch pawl has an extreme end section, movable in a radial direction and projecting toward the can body support means; a punch pawl moving means for moving the extreme ends outwardly in the radial direction; and a dome support means, which abuts the dome section from the inside of the punch pawls, in the radial direction thereof, for supporting the dome section.
In the can manufacturing apparatus, the can bottom processing means expands the extreme ends of the punch pawls by moving relatively, with respect to the can bottom, in the direction of the can axis. In this way, the inner recesses are formed on the inner peripheral wall of the annual projection of the can bottom. Since the inner recesses can be formed while pressing the dome section with the dome support member, the dome section hangs down when the inner recesses are formed.
It is preferable that the can manufacturing apparatus, according to the first aspect of the present invention, is arranged such that the can and the can body support means are movable in the direction of the can axis, with respect to the apparatus main body. The can bottom forming means is unmovable in the direction of the can axis, with respect to the apparatus main body.
The can manufacturing apparatus has a relatively simple arrangement as a whole, even though the can bottom processing means has a relatively complex arrangement and is unmovable with respect to the apparatus main body and the can body support means. This is in part because the can body support means has a relatively simple arrangement and is movable with respect to the apparatus main body.
Preferably, the can manufacturing apparatus, according to the first aspect of the present invention, includes: an outer peripheral wall forming means for forming the outer peripheral wall of the annular projection.
In the can manufacturing apparatus, the inner and outer peripheral walls of the annular projection can be substantially simultaneously formed during the same step.
According to a second aspect of the present invention. A can manufacturing method for manufacturing a can having a dome section and an annular projection. The dome section is formed on the bottom of the can and is recessed inwardly of the body of the can. The annular projection is formed around the peripheral edge of the dome section and projects outwardly in the direction of the can axis. A plurality of inner recesses are formed on the inner peripheral wall of the annular projection in a circumferential direction so as to be recessed inwardly of the can body. The can manufacturing method includes the steps of: supporting the can body; supporting the dome section with a dome support member which abuts the dome section; and forming a plurality of inner recesses, wherein each inner recess is recessed inwardly of the can body on the inner peripheral wall of the annular projection in the circumferential direction. The recessing of the inner recess is accomplished by moving the extreme ends of a plurality of punch pawls. The punch pawls are disposed in the circumferential direction and are movable in a radial direction from a region located inwardly of the nose section of the annular projection at the extreme end thereof in the radial direction to an external region in the radial direction.
According to the can manufacturing method, the inner recesses can be correctly formed in the region inwardly of the nose section of the annular projection in the radial direction by moving the punch pawls outwardly in the radial direction. The punch pawls abut the can bottom, after the annular projection is formed. The recesses are formed independently of the formation of the annular projection. Accordingly, stress in a planar direction and stress in a thickness direction do not simultaneously act on the inner peripheral wall. This results in a prevention of the occurrence of cracks in the inner recesses. Cracks would otherwise occur due to the severe plastic deformation likely to be caused when the inner recesses are formed simultaneously with the annular projection.
According to a third aspect of the present invention, a can is formed having a dome section and an annular projection. The dome section is formed on a bottom of the can and is recessed inwardly of a body of the can. The annular projection is formed around the peripheral edge of the dome section so as to project outwardly in the direction of the can axis. A plurality of inner recesses are formed on the inner peripheral wall of the annular projection in a circumferential direction so as to be recessed inwardly of the can body, wherein each of the inner recesses is curved at a predetermined radius of curvature. An angle xcex8, located between the tangential line at the upper end of each inner recess and the tangential line at the lower end thereof, is set to satisfy the inequality, 83xc2x0xe2x89xa6xcex8xe2x89xa6103xc2x0, and more preferably to satisfy the inequality, 98xc2x0xe2x89xa6xcex8xe2x89xa6103xc2x0.
In the can arranged as described above, a sufficient can strength can be secured by improving the pressure withstanding strength and the falling strength of the can.
In the can according to the third aspect of the present invention, when the shortest radius of the inner peripheral wall is represented by D1 and the longest radius of the dome section is represented by D3, it is preferable that the radii have the relationship of 1.01xe2x89xa6D3/D1xe2x89xa61.15.
In the can arranged as described above, since the inner recesses are formed so that the relationship of 1.01xe2x89xa6D3/D1xe2x89xa61.15 is satisfied, the strength of the can can be more increased.
In the can according to the third aspect of the present invention, it is preferable that the region, where the inner recesses of the inner peripheral wall are formed when viewed in the cross section in the circumferential direction, is 63 to 99% of the entire inner peripheral wall in the circumferential direction.
In the can arranged as described above, the inner peripheral wall portion of the annular projection can obtain a sufficient rigidity by setting the range where the inner recesses is formed to be the above-described region.
Incidentally, the inner recesses can be stably formed, the occurrence of bottom growth can be prevented, and the pressure withstanding strength of the can are improved by forming the inner recesses at locations nearer to the dome section than the portion of the inner peripheral wall. The portion of the inner peripheral wall projects inwardly in the radial direction. The shortest diameter of the inner peripheral wall is formed by forming the inside recesses inwardly of the can body at an angle of 20xc2x0-50xc2x0 with respect to a tangential line extended from the projecting portion in the can axis direction.
According to the third aspect of the present invention, it is preferable that the can have molded sections, which are recessed inwardly of the can body. The molded sections are formed at a portion between the center of the dome section and the inner peripheral wall and number as many as the inner recesses. The molded sections are formed annularly about the center of the dome section.
The can thus formed has an increase can bottom strength and reduced bottom growth.
Incidentally, the strength of the dome section can be increased and stably processed by forming the molded sections to satisfy 0.65xe2x89xa6D2/D1xe2x89xa60.9, where D1 represents the shortest diameter of the inner peripheral wall and D2 represents the annular diameter of the annularly disposed molded sections.